The present invention relates to radar and sonar systems, particularly to reducing range ambiguity in radar and sonar systems, and more particularly to a method and apparatus for reducing range ambiguity in a side-looking or synthetic aperture radar by the use of a main receiver and one or more secondary receivers and subtracting the secondary receiver outputs from the main receiver output so as to cancel the ambiguous signals.
Over the years various radar systems have been developed for different applications. One such radar system now under development at the Lawrence Livermore National Laboratory (LLNL) is the satellite-based Synthetic Aperture Radar (SAR) system being developed for military, strategic, or earth/environmental resources applications. The LLNL SAR system is being designed to have the capability of meeting specified parameters over a wide target area. One of the design constraints on the LLNL-SAR and other SAR systems is the need to keep range ambiguous signals below a specified level. The main source of range-ambiguous signals is the reflection of pulses preceding and following the pulse of interest; i.e., signals from pulses N.+-.1 when pulse N is being reflected from the desired range R.
Normally, range-ambiguous signals are rejected primarily by the response of the SAR antenna(s) as a function of elevation angle. Because the SAR beam is not parallel to the ground, but intersects it at some angle (the grazing angle), different ranges correspond to different elevations as seen by the SAR. The SAR antenna gain is maximum at the center of the target area, and decreases at greater or lesser elevations (and at longer and shorter ranges). Thus, the range ambiguous signal is attenuated both on transmission and reception. Typically, this attenuation is -10 dB or more in each direction. However, to provide this level of attenuation, the interval between pulses must be significantly (typically 2.times.) longer than the theoretical minimum interval set by (2.times. width of the target 1C, where C is the speed of light); i.e., pulse N-1 must be well beyond the "far" edge of the target when pulse N reaches the "near" edge of the target.
This problem is exacerbated if the transmitted beam is much broader than the beam defined by the receive antenna. In this case, ambiguous signal are attenuated primarily by the falloff of the receiver antenna gain, and not by the transmit antenna pattern. Thus, there is a need for a way to effectively reduce range ambiguity without significantly affecting the desired signal.
The present invention is directed to the solution of the above mentioned need, and provides a modified Synthetic Aperture Radar system with reduced sensitivity to range ambiguities. The invention uses secondary receiver channels to detect the range-ambiguous signals and subtract them from the signal received by the main channel without significantly affecting the desired signal. Both desired and range-ambiguous signals are detected by a main receiver and by one or more identical secondary receivers, all connected to a common antenna with two or more feed systems offset in elevation. The subtraction of signals may be done in real time, or the outputs of the receivers may be recorded separately and combined during signal processing. Thus, the present invention relates to radar and sonar systems, particularly to reducing range ambiguity in radar and sonar systems, and more particularly to a method and apparatus for reducing range ambiguity in radar or sonar systems in which the intended target is viewed obliquely, as in side-looking or synthetic aperture radar systems.